Method of pretreating iron surfaces



"Patented Jan. 4, .1938

etc., the

s PATENT OFFICE warrior: or rmi'raaa'rnve raon SURFACES Alexander Kreldl, Vienna, Austria No Drawing. Application December 7, 1938, Serial No. 701,341. In Austria December I, 1932 a cum. (or 91-10) The present invention relates to a method of enameling the surface of metals, such as iron, steel, cast iron etc.

The invention is based on the recognition of 6 two factors independent of each other, viz: For

one thing, it has. been found from an extraordinarily large number of systematically conducted experiments that to achieve a particularly intimate adhesion of enamel to iron, steel, cast'iron,

formation of an intermediate film of spinel-like composition on the iron surface'to be enamelled is essential;

this film may practically be as thin as desired. In general, and without detailed reference to the structures which have been recently found by X-ray experiments,

splnels are compounds which form between monoxides and sesquioxides in a state of incandes- These two spinel components are, of

course, capable of being yielded by iron itself,

but it is found, make special precautions necessary.

just this property which, it has been I have ascertained, surprisingly, that satisfactory adherence can only be obtained if the iron surface to be enamelled, does not contribute from the sesquioxide state to the formation of the iron surface should be protected to the greatest possible extent from oxidizing to ferric oxide.

Obviously, such oxidation would effect a loosening of the iron base, a process which canits train not be remedied or cured, and which brings in (even against the action of the subsequently applied enamel) stripping, rupturing, flaking and other similar'undesirable results.

The technical utilization of these two novel factors constitutes the essence of the present invention, which comprises a method for the enamelling (without using any ground enamel) of iron,

steel, cast iron or the like in which the surface to be enamelled is pretreated in such a manner that a film of the character above defined may form on such surface; while at the same time the simultaneous formation of ferric oxide on the iron base is inhibited to the maxi mum degree.

I find that I accomplish the object of my invention by heating the iron surface'to be enamelled in the presence of a coating containing a metal-oxide-metal-sesquioxide mixture or combination, phere with the addition of flux, in an atmospoor in oxygen in comparison with air. Under these conditions the film formation takes place'with simultaneousfattainment of the desired surface protection. If it be desired to enhance such surface protection, which might be desirable under certain circumstances, suitable measures to that end may be taken as follows:- the heating period is limited to just that short duration which is necessary 'for the. formation of the spinel film, and it has been found in practice that a heating period of but very few minutes sumces. The heating temperature is desirably adjusted 'to between about 750 C. and about 800 C.

As to the heating atmosphere itself, it developed in the course of practical operation, by way of the admixture of generator gas or the like, that the reduction of the oxygen content to approximately one-third of the normal content is'advantageous. If the metals to be enamelled contain adequate quantities of reducing substances, i. e. oxygen-consuming substances, such as carbon, hydrogen, hydrocarbons and the like, then under proper conditions the requisite lowered content of oxygen may be automatically adjusted so that the heating process is then limited to heating under exclusion of air.

The metal surfaces to be enamelled, before their introduction into the furnace, are moistened with the metal compounds by the heating of which the spinel like conversion is to be effected, the moistening being effected either by immersing the surface in a liquid containing these metal compounds, by spraying etc. In order that the spinel him may smoothly and gaplessly adhere to the metal surface, the moistening fluid has preferably added to it a suitable flux, such as boric acid or fluorspar. It will be obvious that appropriate moistening or wetting substances are not only such which act directly as spinel components, but also such compounds out ofwhich the spinel components are capable of forming at the temperature of incandescence. The purpose of my invention would not be served, and indeed would be defeated, if the metal surface to be enamelled were pretreated with the metal compounds of the present invention without incandescence or by heating in an ordinary atmosphere.

As the rust which usually adheres to iron surfaces isan excellent spinel former, it may be made useful and advantageous in the present process by assigning to the rust the role or function of one of the spinel components in the heating action. This will be recognized as an extraordinarily advantageous feature of the present invention because, thanks to the circum stance that rusty surfaces of any kind may be subjected to the process not only without detrimerit but with positive advantage, the present process is the first in the art which permits containing to 100 parts of water,

the enamelling operation to be out without previous rust removal, thus dispensing with the pickling process, and all its attendant evils in this art. The importance and value of this phase 01' the invention, in its simplification, improvement and cheapening of enamelllng procedure cannot be overestimated.

The method 01' my invention may be carried out in practice in a considerable variety oi'ways. For example, unpickled iron ware with or .without rust adhering thereto,'is wetted with a fluid F eaoa, four parts of mo, twenty parts of can,

iorty ilve parts of boric acid and clay, or is wetted with a fluid containlngto 100 parts 01' water ten parts oLFmQz. six parts of mo, twenty parts of Cal a, thirty parts of boric acid and seven parts of clay; and after gentle drying the article is heated seven minutesdn afurnace at a .temperature of from 785 to 795 0., the atmosphere prevailing in the furnace having its oxygen content reduced to one-fourth 111m thereon is enamelled in or one-fifth the normal content by the admixture flve I parts of ten parts of the surface, whereby a rough coating is produced which adapts said metal surface for enamelllng. 2. The method claimed in claim 1, in which said combination comprises substantially the 1'01- lowing proportion oi substances: 5 parts of FezOa.

- water.

4 parts oi N10, 20 parts of cm, 45 parts of boric acid and parts of clay for 100 parts 01' 3. The method claimed in claim 1, in which said combination comprises substantially the following proportion of substances: 10 parts of FezQa, sixparts of MnO, parts CaFa, parts boric acid and? parts of clay to parts of water.

4. The method claimed in claim 1, in which the oxygen content of the atmosphere present during the heating step amounts to one-third of that present in ordinary atmosphere.

' 5. The method claimed in claim 1, in which the heating step is performed at temperatures ranging between 750 and 800 C. v

- 6. The method claimed in claim 1, in which theiron surface to be pre-treated contains rust.

ALEXANDER KREIDL. 

